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Ruby Programming
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Mixin Modules

When a class includes a module, that module's instance methods become available as instance methods of the class. It's almost as if the module becomes a superclass of the class that uses it. Not surprisingly, that's about how it works. When you include a module, Ruby creates an anonymous proxy class that references that module, and inserts that proxy as the direct superclass of the class that did the including. The proxy class contains references to the instance variables and methods of the module. This is important: the same module may be included in many different classes, and will appear in many different inheritance chains. However, thanks to the proxy class, there is still only one underlying module: change a method definition in that module, and it will change in all classes that include that module, both past and future.

module SillyModule
  def hello
    "Hello."
  end
end
class SillyClass
  include SillyModule
end
s = SillyClass.new
s.hello "Hello."

module SillyModule
  def hello
    "Hi, there!"
  end
end
s.hello "Hi, there!"

The relationship between classes and the modules they include is shown in Figure 19.4 on page 243. If multiple modules are included, they are added to the chain in order.

Figure not available...

If a module itself includes other modules, a chain of proxy classes will be added to any class that includes that module, one proxy for each module that is directly or indirectly included.
Ruby Programming
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